Process for producing photographic images with photosensitive materials and products produced thereby

ABSTRACT

THIS INVENTION RELATES TO A PROCESS OF PRODUCING PHOTOGRAPHIC IMAGES USING PHOTOSENSITIVE MATERIALS WHICH HAVE BEEN PRESENSITIZED BY EXPOSURE TO ACTIVATING RADIATION OS THAT THE PHOTOSENSITIVE SUBSTRAT BECOMES SENSITIVE TO WAVELENGTHS OF RADIATION TO WHICH THE SUBSTRATE IS USUALLY NOT SENSITIVE. THE PHOTOSENSITIVE MATERIAL WHICH BECOMES REVERSIBLY ACTIVATED UPON EXPOSURE TO ACTIVATING RADIATION BUT PREFERABLY REMAINS UNCHANGED AFTER SUCH EXPOSURE IS HEREINAFTER REFERRED TO AS A PHOTOCONDUCTOR. THE PRESENSITIZED PHOTOCONDUCTOR IS EXPOSED TO A PATTERN OF DEACTIVATION TO FORM A LATENT IMAGE THEREON CORRESPONDING TO THE PATTERN. FOR EXAMPLE, THE PRESENSITIZED PHOTOCONDUCTOR IS EXPOSED TO A PATTERN OF LGHT TO WHICH THE PRESENSITIZED PHOTOCONDUCTOR IS SENSITIZED TO FORM A LATENT IMAGE AND, AFTER DECAY OF THE LATEN IMAGE, THE PHOTOCONDUCTOR IS FLOODED WITH SIMILAR LIGHT TO FORM A POSITIVE LATEN IMAGE OF THE PATTERN WHICH IS DEVELOPED BY KNOWN METHODS, E.G. USING IMAGE-PRODUCING AGENTS, TO OBTAIN THE VISIBLE IMAGE OF THE PATTERN.

Jan. 26,-; "197 1 G. M. FLETCHER PROCESS FOR PRODUCING PROTOGRAPHIC IMAGES WITH PHOTOSENS'ITIVE MATERIALS AND PRODUCTS PRODUCEDTHEREBY Filed July 13, 1967 ooow j 903 :moav AllSNEIO' INVENTOR. GERALD M .FLETCHER United States Patent 3,558,308 PROCESS FOR PRODUCING PHOTOGRAPHIC IMAGES WITH PHOTOSENSITIVE MATE- RIALS AND PRODUCTS PRODUCED THEREBY Gerald Matthew Fletcher, Arlington, Mass, assignor to Itek Corporation, Lexington, Mass., a corporation of Delaware Filed July 13, 1967, Ser. No. 653,148

Int. Cl. G03c 5/04 U.S. Cl. 96-27 25 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a process of producing photographic images using photosensitive materials which have been presensitized by exposure to activating radiation so that the photosensitive substrate becomes sensitive to wavelengths of radiation to which the substrate is usually not sensitive. The photosensitive material which becomes reversibly activated upon exposure to activating radiation but preferably remains unchanged after such exposure is hereinafter referred to as a photoconductor. The presensitized photoconductor is exposed to a pattern of deactivation to form a latent image thereon corresponding to the pattern. For example, the presensitized photoconductor is exposed to a pattern of light to which the presensitized photoconductor is sensitized to form a latent image and, after decay of the latent image, the photoconductor is flooded with similar light to form a positive latent image of the pattern which is developed by known methods, e.g. using image-producing agents, to obtain the visible image of the pattern.

BACKGROUND OF THE INVENTION (a) Field of invention This invention relates to processes for production of photographic images using presensitized light-sensitive compositions and products obtained thereby.

(b) Description of the prior art The use of media comprising photoconductors for the production of latent images is described in British Pat. 1,043,250. In the patent, the method generally requires the formation on the media of a latent reversible image corresponding to a pattern of activating light and, at a subsequent time, conversion of the latent image to a visible image using liquid redox systems which contact at least the light-activated portions of the media to form a deposit on the media corresponding to the light-activated portions. The product of this aspect of the described process is a negative corresponding to the initial pattern of activating light, which may be used to produce a positive in known manner. A further modification of the described process which results in a positive visible image involves the further step of deactivating those portions of the media which were activated by the initial pattern, after which development leads to the positive.

In the art of electrostatic printing, as described for example in U.S. Pats. 3,041,168 and 3,121,006, the basic principle involves providing a substantially uniform electrostatic charge on the photoconductive coating of the recording element followed by discharge of selected areas in order to produce an electrostatic image, which is then developed using powdered developing material, which process requires special developer materials and costly recording media.

images may be produced utilizing presensitized media comprising a photoconductor. The latent image is produced by exposing the medium to a pattern of deactivation to form a latent image thereon corresponding to the pattern. For example, the deactivation is accomplished by exposing such a presensitized medium to a pattern of light to which the medium is sensitive, decaying the resulting latent image corresponding to the pattern and subsequently flooding the medium with light of similar wavelength. The latent positive image is then developed, preferably substantially immediately after the flooding step for optimum results. The developer is preferably a liquid redox system which on contact with activated portions of the medium deposits a solid, visible residue on the activated portions. If desired, the redox system may be applied to the medium prior to the flooding step in which case the visible positive is obtained on flooding when the medium is still wet. On the other hand, if the redox system is applied to the medium and the medium then dried prior to flooding, the visible positive image is obtained by immersion of the medium in a suitable solvent system.

Optimum results are obtained when the flooding step is conducted with light of wavelength to provide maximum density difference between the visible image and the background of the medium. The determination of optimum wavelengths of light for this purpose is described hereinafter.

DESCRIPTION OF PREFERRED EMBODIMENTS The photoconductor 0r photocatalyst is not limited to any group of compounds but may include both organic and inorganic photosensitive materials. Preferred photoconductors useful in this invention are metal containing photoconductors. A preferred group of such photosensitive materials are the inorganic materials such as compounds of a metal and a non-metallic element of Group VI-A of the Periodic Table 1 such as metal Oxides, such as zinc oxide, titanium dioxide, antimony trioxide, aluminum oxide, zirconium dioxide, germanium dioxide, indium trioxide, chromium oxide, magnesium oxide, thorium oxide, cerium oxide, hydrated potassium aluminum silicate (K Al Si O '2HO) tin oxide (SnO bismuth oxide (Bi O lead oxide (PbO), beryllium oxide BeO, silicon dioxide (SiO barium titanate (BaTiO), tantalum oxide (Ta O tellurium oxide (TeO and boron oxide (B 0 metal sulfides such as cadmium sulfide (CdS), zinc sulfide (ZnS) and tin disulfide (SnS metal selenides such as cadium selenide (CdSe). Metal oxides are especially preferred photoconductors of this group. Titanium dioxide is a preferred metal oxide because of its unexpectedly good results.

Also useful in this invention as photoconductors are certain fluorescent materials. Such materials include, for example, compounds such as silver activated zinc sulfide, Zinc activated Zinc oxide, manganese activated zinc phosphate Zn (PO an admixture of copper sulfide, antimony sulfide (SbS) and magnesium oxide (MgO), and cadmium borate.

Organic photoconductors suitable for use in this invention are, for example, the imidazolidinones, the imidazolidinethiones, the tetraarylazacyclooctatetraenes, and thiazines, such as 1,3-di-phenyl-4,5-bis(p-methoxyphenyl) imidazolidinone 2; 4,5 bis(para methoxyphenyl) imidazolidinone-Z; 4 phenyl 5 (para-dimethylaminophenyl)imidazolidinone 2; 4,5 bis (para-methoxyphenyl)imidazolidenthione 2; 3,4,7,8 tetraphenyl 1,2,5,6- tetraazacyclooctatetracne-2,4,6,8; and methylene blue.

Also useful as photoconductors in this invention are the heteropolyacids such as phosphotungstic acid, phosphosilicic acid, and phosphomolybdic acid.

Periodic Table from Langes Handbook of Chemistry, 9th edition, pp. 5657, 1956.

While the exact mechanism of the present process is not known it is believed that the presensitization, i.e. exposure to activating light, e.g. ultraviolet light, causes the transference of electrons of the photoconductor from the valence band to the conductance band, or at least to some similar excited states whereby the electron is loose ly held, thereby converting the photoconductor from an inactive to an active form. If the photoconductor in the active form is in the presence of an electron-accepting agent, a transfer of electrons will take place between the photoconductor and the electron-accepting agent and the latter will be reduced. Accordingly, a simple test to determine whether materials have a photoconductor effect is to mix the material in question with aqueous silver nitrate. In the absence of light, little, if any, reduction of silver ions should occur. At the same time as exposing the same mixture to light, a control sample of an aqueous silver nitrate solution alone is similarly exposed and if the mixture darkens faster than the control sample, the test material is a photoconductor.

It is evident that the gap between the valence and the conducting band of a compound determines the energy needed to make electron transitions and the light required to provide the needed energy is called bandgap light, as employed herein. The more energy needed, the higher the frequency to which the photoconductor will respond. It is known in the art that electrons may be present in secondary levels within the band gap due to impurities or defects in the structure of the photoconductor. With light of suitable energy, which in this case would be less than the band gap, electrons from these levels could be raised to the conduction band. A typical example of a secondary level due to a defect in the structure would be an F-center (electrons trapped at negative ion vacancies) in an alkali halide crystal. The band gap of KCl is about 8.5 ev. (-1460 A.), but the secondary levels due to F- centers are about 2.4 ev. (5400 A.) below the conduction band. Electrons could be raised to the conduction band with 5400 A. light. An example of an impurity photoconductivity could be ZnS doped with Cu. The band gap of ZnS is about 3.7 ev. (3350 A.), but by doping it with Cu one could introduce some secondary levels which would result in photoconduction due to 4600 A. light.

As is generally known, the activation of photoconductors, i.e. transference of electrons from valance bands to conductance bands, is not permanent but rather the activation decays primarily as a function of time. The decay is apparently due to the loss of electrons in the conductance bands, the electrons reverting to lower energy levels, many reverting to the original valence band and others to energy levels intermediate between the respective bands, i.e. secondary levels, or traps. After decay of the activated photoconductor, the medium retains little, if any, ability to reduce silver ions, or similar metal ion, due to the fact that there are little, if any, electrons in the conductance band. When reference is made herein to a decayed sensitized medium it is intended that the photoconductor is in a state intermediate between the active and inactive states by virtue of the fact that electrons of the photoconductor are in the secondary levels, or traps. When the decayed sensitized medium is exposed to an image pattern of light of wavelength longer than the bandgap light, the energy provided is sufiicient to raise the electrons in the secondary levels to the conductance band, but not sufiicient to raise electrons from the valence band to the conductance band. This results in a latent image on the medium corresponding to the pattern. If the medium were contacted with silver ions, an image corresponding to the pattern would be formed, but it would be a negative image. However, by allowing the thus formed latent image to decay, i.e. the electrons of the conductance band on the light activated portions of the medium revert to lower energy levels, many to the original valence band and some to the secondary levels, or traps, the net effect of such decay is to reduce the number of electrons in the secondary levels or traps of the portions of the medium so activated. By comparison, the remaining portions of the medium which were unaffected by the visible light exposure remain substantially unchanged with respect to the number of electrons in the secondary levels or traps.

Consequently, after decaying the latent image, there are created areas differentiated by way of the frequency of electrons in the secondary levels. The preferred method of decay is to merely allow the medium to stand until decay is substantially complete which is readily determinable by testing a sample with aqueous silver nitrate. When decay is complete, the sample should reduce silver ions to substantially the same extent as a control sample. Other methods of decay may be employed, for example, heating the medium to temperatures of up to about 250 F. Of course, care should be exercised in decay operations to avoid destruction of the decayed sensitization of the medium.

The medium after decay of the image, is then flooded with light of wavelength longer than that of the bandgap light which raises the electrons in the secondary levels of the photoconductor to the conductance band and when the medium is brought into contact with an electron acceptor, electron transfer occurs. Accordingly, if the medium after flooding is contacted with a liquid redox system, reduction of the reducible component thereof occurs. If the reducible component, in the reduced form, is a particulate solid, the result obtained is a visible positive of the decayed latent image.

The foregoing theoretical explanation is oifered to enable a better understanding of the present invention and is believed to reasonably interpret the phenomenon of this invention. Of course, the applicants are not necessarily bound by this explanation.

In the sensitization of the photoconductor it is preferred to utilize high exposure energies of bandgap light. For example, the exposure energy should be about 10 millijoules/cmF. or greater for best results, while energies ranging from 0.05 millijoules/cm. are found operable.

For decaying the photoconductor sensitized with bandgap light it is preferred to allow the medium to stand over a period of time usually of at least about one hour to ensure substantially complete delay. As mentioned hereinbefore, samples of the sensitized medium may be tested using, for example, aqueous silver salt solutions, to determine the time required for decay which will of course be dependent on the photoconductor, the exposure energy, and other factors known to those skilled in the art.

Naturally, after sensitization, the medium should be pro-' tected from exposure to activating light prior to exposure to a visible light pattern to form the latent image and subsequent to this step until the medium is flooded and developed. Techniques for avoiding unintended exposure are well known and need not be enumerated for those skilled in the art.

The exposure of the decayed sensitized photoconductor to the image pattern may be conducted using standard techniques. The light utilized is preferably visible light, of wavelength longer than the bandgap light of the photoconductor. Such wavelength preferably ranges from about 4200 A. to about 7000 A. and may even include the near infrared. Certain ranges are more effective depending on the photoconductor, among other factors, and a minimum of testing will indicate the optimum range for the specific photoconductor. For example, a range from about 4500 A. to about 6000 A. gives excellent results when titanium dioxide is the photoconductor. The time of exposure may be varied considerably, from fractions of a second to several minutes without appreciable variation in the results.

The decay of the pattern latent image is usually permitted to proceed as a function of time, the time periods required being varied. Usually, a wait period of at least 0.5 hours is desirable, but, again, the decay period may be determined by test sampling using silver nitrate solutions as mentioned hereinbefore. When the latent image areas no longer reduce silver more than the background of the medium, the medium is suitable for further treatment to develop a visible positive of the latent image.

As should be obvious to those skilled in the art, the latent image at this stage may be stored almost indefinitely of course protected from unintentional light activation, e.g. in dark storage. As desired, the positive image may be produced, or alternatively, modification of the latent image may be effected, e.g. by erasure, in whole or in part, of the latent image and/or super-imposing of a further image. Such erasure may be accomplished by reactivating the photoconductor with bandgap light.

When it is desired to convert the latent image to a visible positive image, the medium is exposed to light of Wavelength longer than that of bandgap light, e.g. is flooded with such light, and preferably immediately developed. Immediate development while not necessary, minimizes the eifect of decay of activation, for which reason it is preferred. In this respect, it may be advisable, but not necessary, to apply the developer to the medium prior to the flooding step and the visible positive will form on flooding. If the developer requires solvent, the visible image will not form unless solvent is present. Therefore, if the developer including solvent is applied to the medium and the solvent is subsequently removed prior to flooding, exposure of the flooded medium to the solvent will result in image formation.

As indicated herein, the developer may be applied immediately after the flooding step, or at least within a reasonable period of time before appreciable decay of the activation as will be appreciated by those skilled in the art.

The developing agents preferred for this invention are liquid redox systems preferably comprising heavy metal ions such as silver, gold, copper, mercury, and other noble metal ions. British Pat. 1,043,250 fully describes the intended developing agents and processes for developing and fixing for use in this invention and is incorporated herein by reference.

As mentioned hereinbefore, optimum density difference between the visible image and the background of the medium is attainable by selection of light of specific wavelengths. The determination of the optimum wavelength is readily accomplished by a mere comparison of the spectral response of the photoconductor before and after bandgap light radiation. For example, the spectral response curve of normal titanium dioxide is determined by plotting the activation versus the wavelength of light, the activation being measured by the ability of the light-activated photoconductor to reduce silver ions from solution as indicated by the density above fog on the thus-treated medium. If the density above fog is plotted against the wavelength of light, the resulting curve approaches zero density as the wavelength approaches that of visible light.

When the titanium dioxide is sensitized with bandgap light, decayed and exposed to light of longer wavelength than bandgap light at the same exposure energy, the corresponding curve does not approach zero in the visible light region of the curve.

Typical spectral response curves are presented in FIG. I which represents the spectral response of titanium dioxide both before and after ultraviolet sensitization and decay.

Curve A represents the spectral response of titanium dioxide (coated in thin layer on a sheet of paper) after exposure to light of wavelength 3660 A. at an intensity of 36.6;/. watts/cm. for five minutes, after a wait time of 1.5 hours (to decay the activation). Samples of the medium comprising titanium dioxide so treated were then exposed to light of varying Wavelength of exposure energy=l5.3,u joules/cm. and the density above fog measured for each sample after treatment with alcoholic silver nitrate. The curve was plotted on the basis of the density above fog corresponding to the wavelength of light applied.

Curve B represents the normal spectral response of identically treated samples of titanium dioxide sheets with ultraviolet sensitization omitted.

It is obvious from a comparison of the respective curves that light of from about 4500 A. and even lower to 4200 A. could be used to obtain density differences between the image and the background of the medium to obtain a positive image. It is obvious too that with light of wavelength lower than about 5000 A., the image would be of varying shades of gray but the background would be darker by comparison. Above 5000 A., the image should be relatively free of silver deposits while the background is of considerable density. Choice of optimum wavelength for the exposure to and flooding with visible light becomes quite obvious and is deter mined by the desired end result. For absolute image clarity (white or near-white on black), wavelengths in the vicinity of 5000 A. and higher should be employed. Where such considerations are not of paramount interest. any wavelength ranging from 4200 A. up to about 7000 A. could be employed, as practicality dictates.

When visible light of higher exposure energy is utilized, the spectral response curve shifts to the higher wavelengths as is evidenced by curves A and B which are determined in identical manner as curves A and B with the exception that the exposure energy of visible light used is 73.2 joules/cm? At higher exposure energy, the density difference are substantial over a longer and higher range of wavelength values. Optimum wavelengths of visible light should be quite apparent in 'view of the foregoing comments.

For the purpose of the foregoing discussion the curves of the graph represent the image areas and the background of the medium. Curve A corresponds to the area activated by flooding with visible light, i.e. the background, while Curve B corresponds to the area deactivated by exposure to visible light in the image-forming step, i.e. the image area.

Although it is not required, it is generally preferred to utilize light of the same wavelength for the image pattern formation and for the flooding prior to development, particularly when light of optimum Wavelength is employed, to obtain best results.

In the initial sensitizing of the photoconductor, bandgap light is employed. Exemplary photoconductors with corresponding bandgap and absorption edges are listed in Table I.

The initial sensitizing of the photoconductor may also be augmented by the presence of dyes in the photoconductor medium. As is well known, the sensitivity of the semiconductor may be increased by known sensitization techniques such as admixture of dyes with the photoconductor. Dye sensitizing permits the use of light of wavelength longer than the bandgap light of the photoconductor.

When employed as data storage media according to the present invention, the photoconductor materials previously discussed herein can suitably be employed in bulk, e.g., in the form of a continuous layer. When used in image forming processes, the photoconductors are conveniently applied to a suitable backing which may be either porous or non-porous, such as of paper, wood, aluminum, glass and the like. The photoconductor, which is suitably used in the form of finely divided particles, may simply be deposited on the surface of such a backing, or can be deposited on such a backing in a hydrophobic or, preferably, a hydrophilic binder known to those skilled in the art of making radiation sensitive papers, Suitable hydrophobic binders, for example, include the polyvinylacetate resin binders commonly used in the preparation of papers for electrostatic printing processes. Typical of the preferred hydrophilic binders having a limited water solubility are gelatin, polyvinyl alcohol, and ethyl cellulose, for example, though many other materials of both types could be mentioned. Particularly advantageous results are employed when the finely divided photoconductor is merely dispersed in the interstices of a fibrous backing such as paper, the fibers of the backing acting to lock in and to hold the photoconductor particles in the finished structure. For example, the photoconductor is easily incorporated in paper during its manufacture by methods known in the papermaking art.

A better understanding of the invention will be had by reference to the following examples, given by way of illustration of the methods of carrying out the invention.

EXAMPLE 1 A mixture of 4 parts by weight of titanium dioxide and 1 part by weight of an emulsion of polyvinyl alcohol resin containing about 50 percent solids in water is used to coat paper sheets.

A sheet of the coated paper is exposed to light (ultraviolet) of wavelength 3660 A. for five minutes at an intensity of 50p. watts/cmF. The so-treated paper is allowed to decay by dark storage for 30 minutes, after which it is exposed to an image pattern of activating light of wavelength 4800 A. for five minutes at an intensity of 50p. watts/cm}. The latent image pattern is allowed to decay by similar dark storage for one hour.

The sheet is then flooded with light of wavelength 4800 A. at an intensity of 50p. watts/cm. for five minutes and then dipped in a saturated solution of silver nitrate in methanol followed by dipping into a solution comprising g. phenidone, 40 g. of citric acid monohydrate and one liter of methanol. The resulting visible image bearing print is a positive of the latent image pattern which is fixed by immersion in an aqueous solution of sodium thiosulfate followed by washing in running water.

The density difference of the positive is 0.14.

EXAMPLE 2 The procedure of Example 1 is repeated with the exception that the flooding step is conducted with light of wavelength 5700 A. at the same intensity and for the same time of exposure.

The resulting positive image is lighter than that obtained in Example 1. The density difference is 0.22.

EXAMPLE 3 A sheet of the coated paper described in Example 1 is exposed to light of wavelength 3660 A. at an intensity of 50 watts/cm. for 5 minutes and allowed to decay by dark storage for 30 minutes.

The sheet is exposed to an image pattern of light of Wavelength 6000 A. at an intensity of 83.3 watts/cm. for a period of 5 minutes to form a latent image of the pattern, which is allowed to decay by dark storage for one hour.

Flooding of the sheet with light of Wavelength 6000 A. at intensity 83.3 watts/cm. for five minutes following by developing with silver nitrate as described in Example 1 gives a positive visible image corresponding to the image pattern. The density difference is 0.24.

EXAMPLE 4 The procedure of Example 1 is repeated using sheets prepared with an acrylate binder in lieu of polyvinyl alcohol with similar results.

DENSITY DIFFERENCE DETERMINATION In the preceding examples, reference is made to the density difference which is a measure of the contrast between areas of the medium which have been deactivated by the image exposure step and those which were not effected. The determination involves exposing the medium to ultraviolet light as described in the examples, followed by dark storage. The upper half of the medium is then masked and the lower half of the medium is exposed to visible light (which step corresponds to the latent image formation described in the examples). The so-exposed medium is then dark stored to permit decay of the visible light exposure. The masking is then removed and the entire medium is flooded with light and developed. The upper half of the medium is darker than the lower half and the contrast between the two halves of the medium is expressed as the density difference. It follows that the density difference is responsible for production of the positive visible image.

The developer solution employed in the foregoing examples contains silver ion which is preferred. In practice, the developer may include any metallic ion which is at least as strong an oxidizing agent as ionic copper, e.g. gold, mercury, platinum, lead and copper.

In lieu of developing with redox system one may employ resins which are affected by the light activated areas of the photoconductor medium to produce relief images. For example, a resin coating comprised of 15 parts acrylamide to 1 part methylene-bis-acrylamide will be rendered insoluble to water and the soluble resin may be removed by water-washing to medium leaving the image areas of the medium free of resin while the background retains the insolubilized resin, resulting in both a visible and a negative relief image.

The present process also permits use of dyes to sensitize the photoconductor to additional ranges of electromagnetic radiation. Such dyes are well known to the art and include, for example, cyanine dyes, dicarbocyanine dyes, the carbocyanine dyes, and hemicyanine dyes. After the sensitization of the photoconductor, image formation and development, the dye may be removed by dissolving the dye out of the substrate or by contacting with a suitable oxidizing agent. Preferably, the dye may be removed by contacting the medium with a solution of a thionate, e.g. sulfites and/or bisulfites, preferably in the form of salts with alkali or alkaline earth metals. The preferred method of bleaching with thionates is described in commonly owned copending application U.S. Ser. No. 641,126, filed May 25, 1967, the disclosure of which is incorporated herein by reference.

In the image-forming step, customary methods such as projection or contact printing may be utilized. A master is either projected on the medium or printed by contact With the medium using art-recognized procedures.

Since the present invention permits recording of images, e.g. on relatively simple economical recording media by use of ordinary light, i.e. visible light, with storage of the recorded images over long periods of time, it is of special use in data storage systems. A further advantage of the present process resides in the ease of erasure for correction, e.g. by exposure of selected areas of the medium to high intensity of bandgap light of the photoconductor or by any of the art recognized procedures such as application of heat to the areas selected for erasure.

Over printing may then be accomplished for effecting correction of the stored images. In such use, the recording medium would be in the form of a roll of tape or film. Of special significance, is the use of the present process for recording latent images of random events the occurrence of which is not predictable. When it is desired, the latent images of selected events may be rendered visible by developing, or, if none of the random events depict occurrences for which a permanent record is desired, the recording medium may be erased and reused. Even when the medium is developed, it is possible to erase the de veloped images and reuse the medium. To illustrate, the present process may be used in conjunction with a motion picture type camera with intermittent film advance or with a still camera or a camera using either roll or sheet film. The type of camera employed will be determined by the type of events or actions to be recorded and the particular photoconductor medium desired.

The film for such cameras has been pretreated by exposure to activating light, e.g. ultraviolet light, after which it is allowed to decay in dark storage. The film is now sensitive to light of longer wavelength, e.g. visible light, and latent images may be recorded by exposure of the film in the camera. The camera is provided means for advancing the photoconductor medium, e.g. film, as latent images are formed thereon. When the supply of unexposed film is exhausted, the film feed is reversed and the exposed film is fed back to the original supply spool, after erasure of the latent images already formed, while recording further latent images during the feed-back. This process may be continued until it is desired to print certain latent images which can be accomplished at any time by interrupting the sequence before erasure of the selected images. Developing is accomplished by flooding the film with light of longer wavelength as described herein and contacting with developer.

Commonly owned copending application Ser. No. 360,- 006 filed Apr. 15, 1964, now US. Pat. No. 3,390,620, describes suitable apparatus and processes for this use and the disclosure thereof is incorporated herein by reference.

Although preference for liquid redox systems has been mentioned, in general, any image-producing agent may be employed to convert the latent image into a visible image. For example, solid toners may be employed as described in British specification 935,621. In addition, visible images may be produced using charged particles as employed in xerographic developing. As is appreciated by those skilled in the art, the selection of suitable image-producing agents is predicated on the activation of the photoconductor.

If desired, the image-producing agent may be applied to the medium after decay of the initial sensitization, and prior to image exposure as mentioned here-inbefore.

In the step of sensitizing the medium comprising a photo conductor, the use of bandgap light for this purpose has been described as a preferred method. In addition to bandgap light, the medium comprising a photoconductor may also be sensitized by use of X-rays, neutrons and for heat in lieu of bandgap light. The medium after sensitizing by these additional methods of activation is then allowed to decay and is useful in the same manner in producing images as herein described.

What is claimed is:

1. A method of producing a latent image comprising initially:

(a) exposing to activating radiation a copy medium comprising a photoconductor to form a latent image capable of reducing metal ions thereon corresponding to the exposed portions of the medium;

(b) decaying the latent image so formed so that the photoconductor is in a state intermediate between the active and inactive states in that the photoconductor on contact with a solution of metal ions in the absence of activating radiation is substantially non-reducing to thereby produce a decayed copy medium; and

(c) then exposing at least decayed portions of the medium to a pattern of deactivation to erase the re- 5 maining activation of the decayed photoconductor so that the photoconductor is substantially no longer sensitive to light of wavelength longer than bandgap light to form a latent image thereon corresponding to said pattern.

2. A method of producing a latent image comprising the steps of:

(a) exposing to activating radiation a copy medium comprising a photoconductor to form a latent image capable of reducing metal ions thereon corresponding to the exposed portions of the medium;

(b) decaying the latent image so formed so that the photoconductor is in a state intermediate between the active and inactive states in that the photoconductor on contact with a solution of metal ions in the absence of activating radiation is substantially nonreducing to thereby produce a decayed sensitized medium;

() exposing the decayed sensitized medium comprising a photoconductor to a pattern of light of Wavelength longer than the bandgap light of the photoconductor to form a latent image thereon corresponding to said pattern; and

(d) maintaining the medium in an environment substantially free of activating radiation so that the portions of the medium exposed in step (c) substantially lose their sensitivity to light of wavelength longer than bandgap light.

3. A method according to claim 2 including the further step of exposing the medium to light of wavelength longer 35 than the bandgap light of the photoconductor and producing a visible image corresponding to said latent image by contacting the medium with an image-producing agent.

4. A method according to claim 3 wherein the imageproducing agent undergoes an oxidation/reduction type 40 reaction upon contact with an activated photoconductor.

5. A method according to claim 4 wherein the medium is contacted with the image forming material prior to the further exposure of claim 3.

6. A method according to claim 4 wherein the medium 45 is contacted with the image forming material subsequent to the further exposure of claim 3.

7. A method according to claim 1 wherein the photoconductor is a compound of a metal with a non-metallic element of Group VI-A of the Periodic Table.

5 8. A method according to claim 7 wherein the photoconductor is selected from the group consisting of titanium dioxide, zinc oxide, zirconium dioxide, zinc sulfide, lead oxide, silicon dioxide, aluminum oxide, chromium oxide, magnesium oxide, thorium dioxide and cerium dioxide.

5 9. A method of producing a visible image comprising the steps of:

(a) exposing to activating radiation a copy medium comprising a reversibly activatable photoconductor to form a latent image capable of reducing metal ions thereon corresponding to the exposed portions of the medium;

(b) decaying the latent image so formed so that the photoconductor is in a state intermediate between the active and inactive states in that the photoconductor on contact with a solution of metal ions in the absence of activating radiation is substantially non-reducing to thereby produce a decayed sensitized medium;

(c) exposing the decayed sensitized medium comprising a photoconductor to a pattern of light of wavelength longer than the bandgap light of the photoconductor to form a latent image thereon corresponding to said pattern;

(d) maintaining the medium in an environment substantially free of activating radiation so that the portions of the medium exposed in step (c) substan- 1 1 tially lose their sensitivity to light of wavelength longer than bandgap light;

(e) exposing the medium to light of wavelength longer than the bandgap light of the photoconductor to obtain a latent image of said pattern; and

(f) producing a visible image corresponding to said latent image by contacting the medium with an imageproducing agent.

10. A method according to claim 9 wherein the imageproducing agent undergoes an oxidation/reduction type reaction upon contact with an activated photoconductor.

11. A method according to claim 10 wherein the medium is contacted with the image-producing agent prior to step (e) and subsequent to step (d).

12. A method according to claim 10 wherein the medium is contacted with the image producing agent subsequent to step (e).

13. A method according to claim 9 wherein the photoconductor is a compound of a metal with a non-metallic element of Group VI-A of the Periodic Table.

14. A method according to claim 9 wherein the photoconductor is selected from the group consisting of titanium dioxide, zinc oxide, zirconium dioxide, zinc sulfide, lead oxide, silicon dioxide, aluminum oxide, chromium oxide, magnesium oxide, thorium dioxide and cerium dioxide.

15. A method according to claim 9 wherein the photoconductor comprises titanium dioxide.

16. A method according to claim 9 wherein the visible light of step (a) is of a wavelength which provides optimum density difference between the visible image and the background of the medium.

17. A method according to claim 10 wherein the imageproducing agent comprises a metallic ion which is at least as strong an oxidizing agent as ionic copper.

18. A method of producing a visible image comprising the steps of:

(a) exposing to ultraviolet light a copy medium comprising titanium dioxide;

(b) decaying the medium so that the titanium dioxide containing medium in the absence of activating radiation is substantially non-reducing and is sensitized to light of wavelength longer than bandgap light to thereby produce a decayed sensitized copy medium;

() exposing the decayed sensitized medium comprising titanium dioxide to a pattern of light of wavelength ranging from about 4200 A. to about 7000 A. to form a latent image thereon corresponding to said pattern;

(d) maintaining the medium in an environment substantially free of activating radiation so that the portions of the medium exposed in step (c) substantially lose their sensitivity to light of wavelength longer than bandgap light;

(e) exposing the medium to visible light to obtain a positive latent image of said pattern; and

(f) producing a visible image corresponding to said latent image by contacting the medium with an image-producing agent.

19. A method according to claim 18 wherein the visible light of step (c) is of a wavelength which provides a maximum density difference between the visible image and the background of the medium.

20. A method according to claim 18 wherein the visible light of step (c) is of a wavelength ranging from about 4200 A. to about 7000 A.

21. A method according to claim 18 wherein the imageproducing agent undergoes an oxidation/reduction type reaction upon contact with an activated photoconductor.

22. A method according to claim 21 wherein the medium is contacted with the image-producing agent prior to step (e) and subsequent to step (d).

23. A method according to claim 21 wherein the medium is contacted with the image-producing agent subsequent to step (e).

24. A method according to claim 21 wherein the imageproducing agent comprises a metallic ion which is at least as strong an oxidizing agent as ionic copper.

25. A method according to claim 24 wherein the metal is silver.

References Cited UNITED STATES PATENTS 3,152,903 10/1964 Shepard et a1. 96-64 3,380,823 4/1968 Gold 96--27 3,414,410 12/1968 Bartlett et a1. 961X FOREIGN PATENTS 1,043,250 9/1966 Great Britain 961 OTHER REFERENCES Shattuck et al., Postexposure of Latent Electrostatic Images, IBM Tech. Discl., vol. 8, No. 4, September 1965,

GEORGE F. LESMES, Primary Examiner R. E. MARTIN, Assistant Examiner US. Cl. X.R. 

